Thyroid UK
82,821 members97,845 posts

Does it make big difference between plasma and serum TSH/T4?

I found out the lab I now use has new machinery and new reference ranges. Serum TSH and serum ft4 will be plasma samples. TSH used to be 0.40-4 and ft4 10-25. Next time 0.5-4.5 and 10.2-21.

So next time I cannot compare results and see how they have changed as numbers , I should compare the % compared to given range in the past and now?

2 Replies

In any particular machine's tests, it is mandatory to prove equivalence in results of serum and plasma for regulatory permission to be given to sell those tests. Otherwise there must be a statement exactly describing the difference - though such a difference would badly damage a maufacturer's selling. However, since the machine has been changed, there might be a difference in results but only because of the different tests now used. But the plasma and serum values will be changed the same, simply because of the different test development. You can compare your results as follows. In each case, calculate how far up the normal range your results are in both test methods. For example if you are half way up from the bottom of one test, that's a value of 50%. Then compare how far up you are in the other machine's same test. This gives you an idea of comparability by using the reference range placement of your data rather than raw figures.




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